(a) Field of the Invention
The present invention relates to a fluid dynamic pressure bearing, a motor, and a recording medium driving device.
(b) Description of Related Art
Up to now, as this type of fluid dynamic pressure bearing, there are ones with the constructions disclosed in, for example, Japanese Unexamined Patent Publication No. 10-196643 (Patent Document 1) and Japanese Unexamined Patent Publication No. 10-213133 (Patent Document 2).
In this type of fluid dynamic pressure bearing, the gaps between the shaft and housing are filled with working fluid, and the shaft is supported rotatably relative to the housing, while maintaining the shaft and the housing such that they do not make contact with each other, by using dynamic pressure generated by the rotation.
The fluid dynamic pressure bearing in Patent Document 1 is provided with a thrust bearing plate (flange), in which dynamic pressure generating grooves are formed, on one end of a shaft, and is provided with through holes which pass through the thrust bearing plate in the axial direction on the inside, in the radial direction, of a ring area in which the dynamic pressure generating grooves are formed, for the purpose of circulatory supply of working fluid.
Furthermore, the fluid dynamic pressure bearing of Patent Document 2 is provided with a thrust bearing plate (thrust bearing plate section) having a step shaped dynamic pressure generating device on one end of a shaft, and which is provided with through holes which pass through the thrust bearing plate in the axial direction on the inside, in the radial direction, of the dynamic pressure generating device as breather holes for extracting air intermixed in the working fluid.
Moreover, in general, the dynamic pressure generating grooves formed on this thrust bearing plate are formed by a press operation, in which the thrust bearing plate is inserted in the thickness direction, due to considerations of manufacturing efficiency and manufacturing cost of the shaft.
In the case where the dynamic pressure generating grooves are formed on the end faces of the thrust bearing plate by a press operation, if there is no place for the material forming the thrust bearing plate to escape, there is an inconvenience in that dispersion occurs in the depth of the dynamic pressure generating grooves formed. In particular, the material from the parts positioned on the outside in the radial direction of the thrust bearing plate escapes outwardly in the radial direction when it is pressed in the axial direction. However, there is no place for the part positioned inside in the radial direction to escape. As a result, the dynamic pressure generating grooves are formed too shallow. In order to avoid this, it is typical for ring grooves (inner grooves), which are depressed one step lower than the ring area, to be formed inside the ring area on which the dynamic pressure generating grooves are formed, so that the material pressed towards the ring grooves can escape.
In the case where ring grooves are formed on the inner peripheral side of a ring area in which dynamic pressure generating grooves are formed, in the fluid dynamic pressure bearings disclosed in Patent Documents 1 and 2, through holes are formed in the ring grooves. However, in a state in which when the shaft is stopped, the shaft moves in one axial direction with respect to a housing, then regarding the ring area in which the dynamic pressure generating grooves are formed, due to a step difference between the ring grooves and the ring region, the gaps between the shaft and housing are much smaller than that for the ring grooves. Therefore, even if it is possible for working fluid to be supplied from the other end face side of the thrust bearing plate via the through holes provided in the ring grooves, a problem can be envisaged in that the working fluid in the through holes is obstructed by the step difference during start up, making it difficult for it to enter the gap between the shaft and the flange in the ring region.
Furthermore, as described above, during start up from a stopped state when the end face of the thrust bearing plate and the housing inner surface are close together, bubbles are generated in the locations where the absolute quantity of the working fluid is low, due to the negative pressure generated by the working fluid being drawn in by the dynamic pressure generating grooves. The generated bubbles stagnate in a wide range on the ring region when the shaft rotates, but after the shaft stops, they are likely to stagnate in the ring grooves. When bubbles are generated, the lubrication properties of the working fluid diminish, and the rotation becomes unstable. Therefore there are inconveniences of oscillation occurring and the like. Moreover, since the liquid surface of the working fluid rises due to the generation of bubbles, an inconvenience of leakage out of the housing can also be envisaged.